<html>
<meta charset="utf-8">

<head>
    <script src="jquery-3.2.1.min.js"></script>
    <script>
        $(function () {
            $('button:first').click(function () {

                $('div.show_css').load('test.txt');
            });
        })
    </script>
    <style>
        .show_css {
            width: 400px;
            height: 200px;
            background-color: peachpuff;
            color: gray;
            border: 2px solid lightslategray;
        }
    </style>
</head>
<div>
    <div class="show_css">加载内容</div>
    <button>点击加载</button> &nbsp; <a href="http://www.godsuper.com/AJAX-%E5%8A%A0%E8%BD%BD.html">演示地址</a>
</div>

</html>